The endo restorative interface, dcna 2010, ree & schwartz
T h e En d o - R e s t o r a t i v eI n t e r f a c e : C u r ren tConcepts a b,c,Marga Ree, DDS, MSc , Richard S. Schwartz, DDS * KEYWORDS Endodontics Restorative dentistry Adhesive dentistry PostsThe primary goals of endodontic treatment are straightforward: to debride and disin-fect the root canal space to the greatest possible extent, and then seal the canals aseffectively as possible. The materials and techniques change somewhat over time, butnot the ultimate goals. The primary goals of restorative treatment are to restore teethto function and comfort and in some cases, aesthetics. Once again, the materials andtechniques change, but not the ultimate goals of treatment. Successful endodontictreatment depends on the restorative treatment that follows. The connection betweenendodontic treatment and restorative dentistry is well accepted, but the best restor-ative approaches for endodontically treated teeth have always been somewhatcontroversial. The topic is no less controversial today, despite the massive (andever growing) amount of information available from research, journal articles, courses,‘‘expert’’ opinions, and various sources from the Internet. In fact, information overloadcontributes to the controversy because so much of it is contradictory. With the emergence of implants into the mainstream of dentistry, there has beenmore emphasis on long-term outcomes and on evaluating the ‘‘restorability’’ of teethprior to endodontic treatment. Patients are not well served if the endodontic treatmentis successful but the tooth fails. The long-term viability of endodontically treated teethis no longer a ‘‘given’’ in the implant era. In consequence, some teeth that might havereceived endodontic treatment in the past are now extracted and replaced withimplant-supported prostheses if they are marginally restorable or it makes more sensein the overall treatment plan. It is not possible to review in one article all the literatureon the restoration of endodontically treated teeth. This article therefore focusesprimarily on current concepts based on the literature from the past 10 years or so,and provides treatment guidelines based on that research. a Meeuwstraat 110, 1444 VH Purmerend, Netherlands b 1130 East Sonterra Boulevard, Suite 140, San Antonio, TX 78258, USA c University of Texas Health Science Center at San Antonio, San Antonio, TX, USA * Corresponding author. 1130 East Sonterra Boulevard, Suite 140, San Antonio, TX 78258. E-mail address: firstname.lastname@example.org Dent Clin N Am 54 (2010) 345–374 doi:10.1016/j.cden.2009.12.005 dental.theclinics.com 0011-8532/10/$ – see front matter ª 2010 Elsevier Inc. All rights reserved.
346 Ree Schwartz THE RELATIONSHIP BETWEEN ENDODONTICS AND RESTORATIVE DENTISTRY Long-term success of endodontic treatment is highly dependent on the restorative treatment that follows. Once restored, the tooth must be structurally sound and the disinfected status of the root canal system must be maintained. Because microorgan- isms are known to be the primary etiologic factor for apical periodontitis1 and endodontic failure,2 contamination of the root canal system during or after restorative treatment is considered an important factor in the ultimate success or failure. Expo- sure of gutta-percha to saliva in the pulp chamber results in migration of bacteria to the apex in a matter of days.3 Endotoxin reaches the apex even faster.4 The impor- tance of the coronal restoration in successful endodontic treatment has been shown in several studies.5,6 Delayed restoration has been show to result in lower success rates.7 Successful restorative treatment is also greatly influenced by the execution of the endodontic procedures. Radicular and coronal tooth structure should be preserved to the greatest possible extent during endodontic procedures.8–10 Root canal prepa- rations should attempt to preserve dentin in the coronal one-third of the root. There is no reason to prepare a ‘‘coke bottle’’ type of canal preparation (Fig. 1) that weakens the tooth unnecessarily. Access preparations similarly should be made in such a way that cervical dentin is preserved. The roof of the pulp chamber should be removed carefully, preserving the walls of the chamber as much as possible. The chamber walls should be prepared only to the extent that is necessary for adequate access for endodontic treatment. Many, if not most endodontically treated teeth today are restored with adhesive materials. Adhesive materials provide an immediate seal and some immediate strengthening of the tooth. These materials are generally not dependent on gross mechanical retention, so tooth structure can be preserved. The sections that follow discuss basic principles of adhesive dentistry and some of the limitations, pitfalls, and special problems presented by endodontically treated teeth. BONDING TO ENAMEL Enamel is a highly mineralized tissue that is often present along the margins of access preparations of anterior teeth and sometimes in posterior teeth. Effective bonding Fig. 1. This radiograph shows canals prepared with a ‘‘coke bottle’’ design. Excessive dentin was removed in the cervical one-third of the root and the apical preparations are thin.
Endo-Restorative Interface: Current Concepts 347procedures for enamel were first reported in 1955.11 An acid, such as 30% to 40%phosphoric acid, when applied to enamel will cause selective dissolution of theenamel prisms. Microporosities are created within and around the enamel prisms,which can be infiltrated with a low-viscosity resin and polymerized,12 creating resin‘‘tags’’ that provide micromechanical retention and a strong, durable bond. It is impor-tant to prevent contamination of etched enamel with blood, saliva, or moisture that willinterfere with bonding.13 Poorly etched enamel leads to staining at the margins of therestoration.14 A good enamel bond protects the less durable underlying dentin bond.15BONDING TO METAL-CERAMIC AND ALL-CERAMIC RESTORATIONSAccess cavities are often made through metal-ceramic or all-ceramic materials, so at-taining an effective, durable bond is important. Like enamel, the porcelain margins canbe etched (usually with a 1-minute etch of 10% hydrofluoric acid) to create micropo-rosities, which may be infiltrated with resin and polymerized. Application of silane tothe etched porcelain surface enhances the bond.16 Etched ceramic materials forma strong, durable bond with resin.17BONDING TO DENTIN: RESIN-BASED MATERIALSA smear layer is formed when the dentin surface is cut or abraded with hand or rotaryinstruments. The smear layer adheres to the dentin surface and plugs the dentinaltubules; it consists of ground-up collagen and hydroxyapatite and other substancesthat might be present such as bacteria, salivary components, or pulpal remnants.18The smear layer cannot be rinsed or rubbed off,19 but can be removed with an acidor chelating agent. Some dentin adhesives remove the smear layer, whereas otherspenetrate through the layer and incorporate it into the bond. Both approaches maybe used successfully.12 Bonding to dentin is more complex than bonding to enamel or ceramic. Dentin isa wet substrate and restorative resins are hydrophobic (‘‘water hating’’). Dentinconsists of approximately 50% inorganic mineral (hydroxyapatite) by volume, 30%organic components (primarily type 1 collagen), and 20% fluid.20 The wet environmentand relative lack of a mineralized surface made the development of effective dentinadhesives a challenge. The first successful strategy for dentin adhesion was reported by Nakabayashi andcolleagues in 1982.21 Their ideas were not widely accepted until later in the decade.Nakabayashi showed that resin could be bonded to dentin by demineralizing thedentin surface and applying an intermediate layer that would bond to dentin andrestorative materials. Although not as durable and reliable as enamel bonding, dentinbonding forms the foundation for many of today’s restorative procedures. Nakabaya-shi’s technique was later simplified by combining some of the steps.THE LIMITATIONS OF DENTIN BONDINGFrom the restorative literature it is known that dentin bonding materials have limita-tions, many of which are related to polymerization shrinkage. When resin-based mate-rials polymerize, individual monomer molecules join to form chains that contract as thechains grow and intertwine, and the mass undergoes volumetric shrinkage.22 Resin-based restorative materials shrink from 2% to 7%, depending on the volume occupiedby filler particles and the test method.23–25 The force of polymerization contractionoften exceeds the bond strength of dentin adhesives to dentin, resulting in gap
348 Ree Schwartz formation along the surfaces with the weakest bonds.26 Resins, even in thin layers, generate very high forces from polymerization contraction.27,28 Another limitation of dentin bonding is deterioration of the resin bond over time. This process is well documented in vitro15,29–31 and in vivo.32,33 Loss of bond strength is first detectable in the laboratory at 3 months.30 Interfacial leakage increases as the bond degrades.22,34 Functional forces have been shown to contribute to the degrada- tion of the resin bond in restorative applications.30,35 THE LIMITATIONS OF BONDING IN THE ROOT CANAL SYSTEM The root canal system has an unfavorable geometry for resin bonding.36 Configuration factor or C-Factor, the ratio of bonded to unbonded resin surfaces,23 is often used as a quantitative measure of the geometry of the cavity preparation for bonding. The greater the percentage of unbonded surfaces, the less stress is placed on the bonded surfaces from polymerization contraction. The unbonded surfaces allow plastic defor- mation or flow within the resin mass during polymerization.23,37 A Class 4 cavity prep- aration, for example, has a favorable geometry with a ratio of less than 1:1. There are few if any walls that directly oppose each other, and more than half of the resin surfaces are not bonded. In the root canal system the ratio might be 100:1,23 because virtually every dentin wall has an opposing wall and there are minimal unbonded surfaces. Any ratio greater than 3:1 is considered unfavorable for bonding.38 Because of this unfavorable geometry, it is not possible to achieve the gap-free interface with current materials. Interfacial gaps are virtually always present in bonded restorations in restorative dentistry,39 obturating materials,40 and bonded posts,41,42 and gap formation increases with time.43 THE POTENTIAL PROBLEMS OF USING ADHESIVE MATERIALS DEEP IN THE ROOT CANAL SYSTEM Performing the bonding steps is problematic deep in the root canal system. Uniform application of a primer or adhesive can be difficult. Once the primer is applied, the volatile carrier must be evaporated. This process can also be problematic deep in the canal. If the acetone or alcohol carrier is not completely removed, the bond is adversely affected.44 An in vitro post study by Bouillaguet and colleagues45 reported lower bond strengths were achieved bonding in the root canal system than bonding to flat prepared samples of radicular dentin. COMPATIBILITY PROBLEMS WITH DUAL-CURE AND SELF-CURE RESINS Because penetration with a curing light is limited in the root canal system, dual-cure or self-cure resin adhesives must be used. Dual-cure resins contain components that provide rapid light polymerization in those areas where the curing light penetrates effectively and a slower chemical polymerization in those areas where the light is not effective. Adhesives and sealers that contain a self-cure component have compat- ibility problems with self-etching dentin adhesive systems (ie, sixth and seventh genera- tion), so they should be used with ‘‘fourth generation’’ etch-and-rinse adhesives.41,46,47 IRRIGATING SOLUTIONS AND MEDICAMENTS Sodium hypochlorite is commonly used as an endodontic irrigant because of its anti- microbial and tissue dissolving properties. The antimicrobial properties of sodium hypochlorite are largely due to it being a strong oxidizing agent, but as a result it leaves
Endo-Restorative Interface: Current Concepts 349behind an oxygen-rich layer on the dentin surface. The same applies to chelating agentsthat contain hydrogen peroxide. Oxygen is one of the many substances that inhibitthe polymerization of resins. When dentin bonding agents are applied to an oxygen-rich surface, low bond strengths are achieved48–50 and microleakage is increased.51A reducing agent, such as ascorbic acid and sodium ascorbate, applied to the dentinsurface will reverse the negative affects of sodium hypochlorite.48,51 A final soak withethylenediamine tetra-acetic acid (EDTA) has also been reported to be effective.52BASIC PRINCIPLES FOR RESTORING ENDODONTICALLY TREATED TEETHAlthough many aspects of the restoration of endodontically treated remain controver-sial, there are several areas of general agreement. One of the best documented prin-ciples is cuspal coverage. Several studies evaluated factors that affected the survivalof endodontically treated teeth. Cuspal coverage was the most consistent finding.53–55In one study, teeth with cuspal coverage had a 6 times greater survival rate than teethwithout cuspal coverage.56 Another study showed teeth without cuspal coverage hadonly a 36% survival rate after 5 years.57 Another important principle is preservation of tooth structure. Coronal tooth struc-ture should be preserved to support the core buildup.9,10 Several studies identifyremaining coronal tooth structure as the most important factor in tooth survival in teethwith posts.8,9,58 As stated previously, radicular tooth structure should also be preserved. For mostteeth that are to receive posts, no additional dentin should be removed beyondwhat is necessary to complete the endodontic treatment. If a tooth is prepared fora 0.06 tapered preparation, a 0.06 tapered post should ‘‘drop right in’’ withoutremoving additional radicular dentin. There is wide general agreement that the ‘‘ferrule effect’’ is important. In dentistry,the ferrule refers to the cervical tooth structure that provides retention and resistanceform to the restoration and protects it from fracture. In one study, teeth with a ferrule of1 mm of vertical tooth structure doubled the resistance to fracture compared withteeth restored without a ferrule.59 Other studies have shown maximum beneficialeffects from a ferrule of 1.5 to 2 mm.60–62 The ‘‘ferrule effect’’ is important to long-term success when a post is used.61 In anterior teeth, the lingual aspect of the ferruleis the most important part.63 If the height of the remaining dentin is not sufficient tocreate an adequate ferrule, crown lengthening, orthodontic extrusion, or extractionmay be indicated.TEETH RESTORED WITH POSTSEndodontically treated teeth often have substantial loss of tooth structure and requirea core buildup. If retention and resistance of the core are compromised, a post mayalso be necessary. Custom cast posts and cores or prefabricated metal posts werethe standard for many years. In the past 10 years or so, fiber-reinforced compositeposts have gained popularity.INDICATIONS FOR A POSTThe primary function of a post is to retain a core in a tooth with extensive loss ofcoronal tooth structure.64 Posts should not be placed arbitrarily, however, becausepreparation of a post channel adds a degree of risk to a restorative procedure: Disturbing the seal of the root canal filling, which may lead to microleakage65,66
350 Ree Schwartz Removal of sound tooth structure, which weakens the root and may result in premature loss due to root fracture67,68 Increased risk of perforation.69 Some studies report higher failure rates in endodontically treated teeth with posts than without.7,70 The finding was not universal, however.71 Traditional thought has been that posts do not ‘‘reinforce’’ the root; this was appar- ently true for metal posts,72,73 but there is a growing body of evidence that fiber posts may strengthen the root and make it more resistant to fracture. To date, 9 studies have shown a strengthening effect74–82 while 3 have shown no effect.10,83,84 Metal posts have a high modulus of elasticity, which means that they are stiff and able to withstand forces without distortion. When a force is placed on a tooth contain- ing a stiff post, it is transmitted to the less rigid root dentin, and concentrates at the apex of the post. Stress concentration in the post/root complex increases the chances of fracture. To overcome the concerns about unfavorable stress distribution generated by metal posts, fiber-reinforced composite resin posts were introduced in 1990, with the aim of providing more elastic support to the core. The reduced stress transfer to tooth struc- ture was claimed to reduce the likelihood of root fracture.85 Posts made of materials with a modulus of elasticity similar to dentin are more resilient, absorb more impact force, and distribute the forces better than stiffer posts.36 TYPES OF POSTS Posts can be categorized by modulus of elasticity, composition, fabrication process, shape, and surface texture. Rigid Post Systems Metal – custom cast – prefabricated Zirconium and ceramic. Posts traditionally were made of metal, and were either custom cast or prefabri- cated. Custom cast posts and cores are made of precious or nonprecious casting alloys; prefabricated posts are typically made of stainless steel, nickel chromium alloy, or titanium alloy. With the exception of the titanium alloys, they are very strong. Parallel metal posts are more retentive than tapered posts86 and induce less stress into the root, because they have less wedging effect and are reported to be less likely to cause root fractures than tapered posts.59,87 In a retrospective study, Sorensen and Martinoff 53 reported a higher success rate with parallel metal posts than tapered posts. Tapered posts, on the other hand, require less dentin removal because most roots are tapered. Prefabricated posts can be further divided in active or passive posts. Most active posts are threaded and intended to engage the walls of the canal, whereas passive posts are retained primarily by the frictional retention of the luting agent. Active posts are more retentive than passive posts, but introduce more stress into the root than passive posts.88 Active posts have very limited indications, and are only recommen- ded when the need for retention is the overriding factor. One factor that has reduced the use of metal posts is aesthetics. Metal posts may be visible through translucent all-ceramic restorations, and even with less translucent restorations may cause the marginal gingiva to appear dark. These concerns have led
Endo-Restorative Interface: Current Concepts 351to the development of posts that are white or translucent. Among the materials usedfor ‘‘aesthetic’’ posts are zirconium and other ceramic materials. These posts will workclinically, but have several disadvantages. Among rigid posts, zirconium is stiffer and more brittle than metal. Zirconium postswere shown to cause significantly more root fractures than fiber posts in vitro.89,90When compared with custom cast and fiber posts, ceramic posts had a lower failureload in vivo91 and in vitro.92–94 As a group, they tend to be weaker than metal posts, soa thicker post is necessary, which may require removal of additional radicular toothstructure. Zirconium posts cannot be etched, therefore it is not possible to bonda composite core material to the post, making core retention a problem.92 Retrievalof zirconium and ceramic posts is very difficult if endodontic retreatment is necessaryor if the post fractures. Some ceramic materials can be removed by grinding away theremaining post material with a bur, but this is a tedious and risky procedure. It isimpossible to grind away a zirconium post. In many cases, excessive removal ofdentin is necessary to remove a zirconium post. For these reasons, ceramic and zirco-nium posts should be avoided. Metal and zirconium posts are all radiopaque and clearly visible on a radiograph(Figs. 2 and 3). The radiopacity of titanium is similar to that of gutta-percha, and there-fore sometimes the presence of a titanium post is difficult to detect on radiographs(Fig. 4).Fig. 2. Radiographic appearance of custom cast metal posts.
352 Ree Schwartz Fig. 3. Radiographic appearance of zirconium posts. Nonrigid Post Systems: Fiber Posts Carbon fiber Glass fiber Quartz fiber Silicon fiber. The first composite reinforced fiber posts were made with carbon fibers, which were arranged longitudinally and embedded in an epoxy resin matrix.85 The black carbon fibers were rapidly replaced by more esthetic white and translucent glass and quartz fibers, which are now the standard components in fiber posts. These posts are commonly used in aesthetically demanding areas. The main advantage of fiber posts is the uniform distribution of forces in the root, which results in fewer catastrophic failures than occur with metal posts if an adequate ferrule is present.95 Several in vitro studies report that teeth restored with nonrigid posts have fewer catastrophic, irreparable root fractures when tested to failure.96,97 Clinical studies of fiber post systems also report successful multiyear service with few or no root fractures.8,98,99 A retrospective clinical study of carbon fiber posts and custom cast posts reported root fractures in 9% of teeth restored with cast posts, and no root fractures in teeth restored with fiber posts after 4 years.100 In a long-term retrospective study of the clinical performance of fiber posts by Ferrari and colleagues,8 a 7% to 11% failure rate was reported for 3 different types of fiber posts after a service period of 7 to 11 years. Half of the failures were classified as endodontic failures, the other half were mechanical failures. Out of 985 posts evaluated, the nonen- dodontic failures consisted of one root fracture, one fiber post fracture, 17 crown
Endo-Restorative Interface: Current Concepts 353Fig. 4. Radiographic appearance of a titanium post. Note that the radiopacity of gutta-percha and titanium is very similar.dislodgements, and 21 failures due to post debonding. The mechanical failures werealways related to the lack of coronal tooth structure. In a review by Dietschi andcolleagues101 it was concluded that nonvital teeth restored with composite resin orcomposite resin combined with fiber posts currently represent the best treatment option. Although fiber posts offer several advantages, they do have limitations. Posts andcore foundations are subjected to repeated lateral forces in clinical function. Becausenonrigid posts have a modulus of elasticity and flexural strength close to that of dentin,they flex under occlusal load. When there is an adequate ferrule, the cervical toothstructure itself resists lateral flexion.95 However, in structurally compromised teeththat lack cervical stiffness from dentin walls and an adequate ferrule, a flexible postmay result in micro-movement of the core and coronal leakage,102,103 which in turnmay lead to caries or loss of the core and crown. Fiber posts were shown to lose flexural strength if they are submitted to cyclicloading or to thermocycling104,105 due to degradation of the matrix in which the fibersare embedded. The strength of fiber posts varied between brands, but was directlyrelated to post diameter and was reduced by thermocycling.106 Parallel fiber posts are more retentive than tapered posts.107,108 However, in a clin-ical study by Signore and colleagues99 no difference was found in the long-termsurvival rate of maxillary anterior teeth restored with tapered or parallel-sided glass-fiber posts and full-ceramic crown coverage. The overall survival rate was reportedto be 98.5%. Most fiber posts are relatively radiolucent and have a different radio-graphic appearance than traditional metal posts (Fig. 5). It has been shown that the retention of fiber posts relies mainly on mechanical (fric-tional) retention rather than bonding, similar to metal posts.41,42,109,110 Several in vitro
354 Ree Schwartz Fig. 5. Radiographic appearance of a glass-fiber post. The post is radiolucent, but the radi- opaque composite clearly reveals its outline. studies have confirmed the presence of gaps in the interface between the luting composite resin of the fiber post and the root canal wall,42,110 and that the bond strengths between fiber posts and dentin are low, about 5 to 6 MPa.109,111 This situ- ation is due primarily to the unfavorable bonding environment of the root canal system, as discussed earlier. POST LENGTH AND REMAINING ROOT CANAL FILLING The length of a post is dictated by several factors, some of which are conflicting. Most of the studies on optimum post length were done with metal posts, but there is no compelling evidence that the principles of post length are different for fiber posts. Braga and colleagues112 evaluated the force required to remove glass fiber and metallic cast posts with different lengths. Irrespective the post type, posts with 10-mm length had higher retention values than posts with 6-mm length. In a study by Buttel and colleagues,113 teeth restored with glass-fiber posts with insertion ¨ depths of 6 mm resulted in significantly higher mean failure than teeth with post space preparation of 3 mm. The retention of fiber posts was shown to be directly proportional to the insertion length in resin cubes.114 Several ‘‘rules’’ have been suggested for passively fitting posts: The post length below the alveolar crest should be at least equal to the length above the alveolar crest.64,115 Sorensen and Martinoff 53 reported 97% success if post length at least equaled the crown height. The post should end halfway between the crestal bone and the root apex.64 A post should extend at least apical to the crest of the alveolar bone.67
Endo-Restorative Interface: Current Concepts 355 Another factor that influences post length is the length of the remaining apical rootcanal filling. Several studies have investigated apical seal following post space prep-aration and have reported that 3 to 5 mm of gutta-percha is the minimum recommen-ded,116–118 and longer is better117,118; this is sometimes dictated by the length of thecanal. Post placement in a long root, for example, a canine of 28 mm, allows moreapical root canal filling, as placing a 23-mm post is unnecessary. When using the crite-rion that the post should extend beyond the apical crest, teeth with bone loss needlonger posts than teeth with normal bone height.LIGHT-TRANSMITTING FIBER POSTSAlthough it seems logical that translucent posts would transmit light for enhancementof cure deeper in the canal, there seems to be no consensus in the literature on thisissue. The use of a light-transmitting translucent fiber post was reported to increasethe depth of resin cure in several in vitro studies,119–121 but other studies reportedminimal or no benefits from translucent posts. One study evaluated the influence offiber-post translucency on the degree of conversion of a dual-cure composite. Lowdegrees of conversion were found for the medium and deep depths.122 Another in vitrostudy measured light transmission through 4 different posts of a standard length of 10mm. All posts evaluated showed some light transmission capacity, but with valueslower than 40% of incident light. One post demonstrated less than 1% light transmis-sion.108 Goracci and colleagues evaluated the light transmission of several fiber posts.These investigators reported no light transmission through 2 posts, and for all otherposts light intensity decreased from coronal to apical, and rose again at the apicaltip. Light transmission was significantly higher at the coronal level.123 Another studyshowed that even without a post, the luminous intensity inside the canal decreasedto levels that are insufficient for polymerization, especially in the apical third.124 Basedon these findings, the use of light-cured resin cements for post placement cannot berecommended. The benefits of light-transmitting posts are unclear.IS THERE BENEFIT TO PLACING A POST AFTER ENDODONTIC TREATMENTOF A TOOTH WITH A CROWN?In most cases, when preparing endodontic access through a crown there is no way ofknowing the amount or strength of the underlying tooth structure, which is a particularconcern in small teeth and bridge abutments. When an access preparation is made through a crown, retention is lost.125 When theaccess opening is restored with amalgam or composite resin, the retention values arerestored.125,126 When the access opening is restored with a post, the retention isgreater than before the access was prepared.125 There is growing evidence that the insertion of a fiber post can also increase fractureresistance of teeth with crowns. An in vitro study has shown that placement of fiberposts can improve fracture resistance in maxillary premolars under full-coveragecrowns.76 The use of fiber posts in endodontically treated maxillary incisors withdifferent types of full-coverage crowns increased their resistance to fracture81,82and improved the prognosis in case of fracture.81 The type of crown was not a signif-icant factor affecting fracture resistance, whereas the presence of a post was. D’Ar-cangelo and colleagues80 showed that fiber posts significantly increased mean loadvalues for maxillary central incisors prepared for veneers. Based on these findings, it seems retention will be enhanced by a post, and fractureresistance will probably be improved as long as no additional tooth structure is
356 Ree Schwartz removed. The authors routinely place fiber posts in bridge abutments and small teeth with crowns (Fig. 6). POST PLACEMENT Advantages of Immediate Post Placement The literature on the timing of the post space preparation is inconclusive. Some studies showed less leakage after immediate post space preparation,127,128 whereas other articles showed no difference 118,129 Some in vitro studies showed that delayed cementation of a fiber post resulted in higher retentive strengths.130,131 Scanning elec- tron microscopy examination revealed a more conspicuous presence of sealer remnants on the walls of immediately prepared post spaces.131 Remnants of sealer and gutta-percha may impair adhesive bonding and resin cementation of fiber posts.132,133 Therefore, it is important to clean the root canal walls before conditioning the dentin for post placement. Acid-etching of the prepared post space and EDTA irri- gation combined with ultrasonics are reported to be an effective method.134,135 The use of magnification can facilitate inspection of the post space for cleanliness. Immediate preparation for post placement following obturation has several advan- tages. The operator has a great familiarity with the root canal morphology, working lengths, and reference points of the root canal system. In addition, placement of a temporary post and restoration can be avoided, as maintaining the temporary seal can be difficult. In vitro studies by Fox and Gutteridge136 and by Demarchi and Sato137 showed that teeth restored with temporary posts leaked extensively. LUTING FIBER POSTS Fiber posts are usually luted with lightly filled composite resins. Light penetration is limited, so dual-cure of self-cure luting resins must be used. Some luting resins are used with a separate etchant and primer (total-etch method), whereas others contain an acidic primer in the luting cement (self-etching method). More recently a third cate- gory has been added (self-adhesive method), in which there is no etching and no primer. Several studies have evaluated these luting cements. Goracci and colleagues138 reported that the values achieved by total-etch method were significantly higher than self-etch resin cements. Transmission electron Fig. 6. The authors routinely use fiber posts when restoring access openings through crowns on bridge abutments or small teeth.
Endo-Restorative Interface: Current Concepts 357microscopy analysis revealed that the acidic resin monomers responsible forsubstrate conditioning in self-etch and self-adhesive resin cements did not effectivelyremove the thick smear layer created on root dentin during post space preparation.Valandro and colleagues139 similarly concluded that more reliable bond strengths inthe dowel space might be achieved when using total-etch adhesive systems insteadof self-etching adhesives. A study by Radovic and colleagues140 concluded that theuse of self-etching resin luting systems offer less favorable adhesion to root canaldentin in comparison with the total-etch and self-adhesive approaches. Self-adhesive cements were introduced in 2002 as a new subgroup of resincements. Self-adhesive cements do not require any pretreatment of the toothsubstrate. The cement is mixed and applied in a single clinical step. The applicationof self-adhesive cements to radicular dentin does not result in the formation of hybridlayer or resin tags.138 Luhrs and colleagues141 found the shear bond strength of self- ¨adhesive resin cements to be inferior compared with conventional composite resincements. The sealing ability of 2 self-adhesive resin cements was shown to be signif-icantly lower than a self-etching and 2 conventional dual-cure resin cements. Theinvestigators concluded that although the bonding effectiveness of self-adhesivecements seems promising, their interaction with root dentin might be too weak to mini-mize microleakage at the post-cement-dentin interface.142 In another study by Vro-chari and coworkers, the degree of cure of 4 self-etching or self-adhesive resincements in their self-curing mode was very low. The values obtained in the dual-curingmode were also low.143 Self-adhesive cements offer a new, simpler approach, but the efficacy of manyrecently marketed products is not known, and there are few data in the literatureregarding their in vitro or clinical performance. At this point in their development,the literature generally shows them to be inferior to the total-etch method.THE POST/RESIN INTERFACEIn addition to the interface between the resin cement and dentin, the post/resin inter-face is also important. Several surface treatments of the post have been recommen-ded for improving the bonding of resin cements or core materials to fiber posts.Silane ApplicationThe literature is mixed on the value of application of silane to fiber posts. In one study,pretreatment of fiber posts with silane did not result in an enhanced bonding betweenpost and 6 different resin cements144 and the effect of silanization was reported to beclinically negligible.145,146 Perdigao and colleagues147 showed that the use of a silane ˜coupling agent did not increase the push-out bond strengths of 3 different fiber posts.On the contrary, Goracci and colleagues148 reported an improvement in bond strengthbetween silanized fiber posts and flowable composite cores. Aksornmuang andcolleagues149 similarly confirmed the benefit of silane application in enhancing the mi-crotensile bond strength of a dual-cure resin core material to translucent fiber posts.Air AbrasionIt is well accepted that sandblasting with alumina particles results in an increasedsurface roughness and surface area, but it also provided mixed results when usedwith fiber posts. A study by Valandro and colleagues150 showed that air abrasionwith silica-coated aluminum oxide particles, followed by silanization, improved thebond strength between quartz fiber posts and resin cements. Sandblasting wasalso shown to improve the retention of fiber posts in 2 other studies.151,152 The
358 Ree Schwartz mechanical action of sandblasting probably removes of the superficial layer of resinous matrix, creating micro-retentive spaces on the post surface. On the other hand, Bitter and colleagues144 reported little influence of sandblasting on the bond strength between fiber posts and resinous cements. Sahafi and colleagues106 evalu- ated the efficacy of sand blasting the surface of zirconium and fiber posts with silica oxide. Despite the satisfactory bond strengths, the treatment was considered too aggressive for fiber posts, with the risk of significantly modifying their shape and fit within the root canals. Air abrasion should be used with caution, as it is difficult to stan- dardize the procedure. Alternative Etching Techniques Hydrogen peroxide and sodium ethoxide are commonly employed for conditioning epoxy resin surfaces. The etching effect of these chemicals depends on partial resinous matrix dissolution, breaking epoxy resin bonds through substrate oxida- tion.153 A similar approach has been proposed for pretreatment of fiber posts to increase their responsiveness to silanization, achieving satisfactory results for both chemicals.154,155 The conditioning treatment consisted of fiber posts immersion in the solutions for 10 to 20 minutes. By removing a surface layer of epoxy resin, a larger surface area of exposed quartz fibers is available for silanization. The spaces between these fibers provide additional sites for micromechanical retention of the resin composites. Similar results were obtained by pretreating methacrylate-based posts with either hydrogen peroxide or hydrofluoric acid.156 Pretreatment with 24% H2O2 for 10 minutes, followed by silane application, seems to be a clinically feasible, inexpensive, and effective method for enhancing interfacial strengths between both methacrylate-based and epoxy resin-based fiber posts and resin composites.155,156 Pretreatment with H2O2 can be performed well in advance of the clinical use. CLINICAL PROCEDURES FOR FIBER POST CEMENTATION AND CORE BUILDUP As discussed earlier, there are a lot of advantages to immediate post placement after finishing the endodontic treatment. The use of rubber dam, magnification, and good illumination are essential to carry out root canal treatment to a consistently high stan- dard. Similar conditions are also required for all clinical procedures involving an adhe- sive bonding. Gutta-percha can be removed with the aid of heat or chemicals, but most often the easiest and most efficient method is with rotary instruments. If the clinician who has performed the root canal treatment is going to place the post as well, obturation can be completed only in the apical portion of the canal. There is a direct correlation between the diameter of the fiber post and fracture strength.157 Buttel and colleagues113 showed that post fit did not have a significant ¨ influence on fracture resistance, irrespective of the post length. Their results suggest that excessive post space preparation aimed at producing an optimal circumferential post fit is not required to improve fracture resistance of roots. All remnants of gutta-percha, Resilon, sealer, and temporary filling materials should be removed using small micro-brushes with alcohol or a detergent. Acid-etching of the post space and an EDTA irrigation combined with ultrasonics are effective in obtaining a clean post space.134,135 Air abrasion is an effective way the clean the pulp floor. The use of a matrix helps confine the core material, enhances the adaptation of the composite to the remaining tooth structure and post, and prevents bonding core material to adjacent teeth. However, the use of a matrix is not essential.158
Endo-Restorative Interface: Current Concepts 359 As discussed earlier, the use of a fourth-generation, 3-step etch-and-rinse adhe-sive with self-cure and dual-cure composites is recommended. If a self-etchingadhesive is used, no rinsing takes place, which might result in dentin walls thatare less clean. Moreover, when using a ZOE sealer, a self-etching adhesive incorpo-rates Eugenol in the hybrid layer, which inhibits the polymerization of resins. Afterthe etch-and-rinse step, paper points are recommended to dry the canal beforethe application of the primer and adhesive. The use of small micro-brushes hasbeen shown to promote higher bond strength values than other brushes tested.159In the same study, the use of paper points to remove excess adhesive resulted inhigher bond strengths. A self-cure or dual-cure resin composite may be used rather than a separate lutingcement for cementation of the post and the subsequent buildup. These compositesmay be bulk-filled because they do not require deep penetration with a curing light.Self-cure and dual-cure composites polymerize more slowly than light-cure materials,allowing the material to flow during polymerization contraction, and placing less stresson the adhesive bond.24 To minimize void formation, the composite is injected into the conditioned postchannel using a syringe with a specially designed small tip, a so-called needle tube.The tip is inserted until it reaches the coronal part of the root canal filling, and isthen applied from the base of the post channel coronally until the post space is filledto the brim. Then the pretreated post is immediately inserted into the composite fillingthe post space, without the need to further cover the post itself with composite.Finally, the composite core is added to the newly placed post, using the same self-cure or dual-cure composite applied into the post space. This procedure can bedone immediately or after the composite in the post channel has completely set. AFig. 7. Mandibular second premolar is treatment planned for an endodontic retreatment,post, core, and crown.
360 Ree Schwartz Fig. 8. Cone-fit. Fig. 9. Obturation is complete and the post channel is free of remnants of root canal filling. The obturating material is seen at the base of the post channel.
Endo-Restorative Interface: Current Concepts 361light-cured composite may also be used for the buildup. It is critical that the post isfully embedded in composite to avoid the uptake of moisture, which may compromiseits mechanical properties.160–162 Embedding can be obtained by cutting back the posta few millimeters below the cavo-surface margin before placement or after thecomposite of the core has completely set. If a matrix has been used, the core needsto be contoured and the occlusion needs to be adjusted. Another option is to completethe crown preparation at the same session.CLINICAL SEQUENCE 1. Isolate the tooth with rubber dam and carry out root canal treatment (Figs. 7 and 8). 2. Remove all remnants of root filling and temporary filling materials using small micro-brushes with alcohol (Fig. 9). 3. Clean the floor with air abrasion. 4. Select a post that passively fits into the available canal space (Fig. 10). 5. Pre-fit the post and cut it back at the coronal or apical end to accommodate the existing post channel. In oval shaped canals, or premolars with 2 canals, consider placing 2 posts. 6. Confirm the fit of the post with a radiograph if necessary. 7. Air abrade the post surface with 50-mm alumina particles for 5 seconds, or use a pretreated post that has been immersed in 24% H2O2 for 10 minutes. Clean the post surface by acid-etching the surface with 37% phosphoric acid, rinse and air-dry. 8. Apply silane to the post surface according to the manufacturer’s instructions.Fig. 10. The largest post that fits passively in the available post space is selected. After fin-ishing the root canal treatment, no additional dentin is removed to accommodate the post.
362 Ree Schwartz Fig. 11. Phosphoric acid 37% is applied to the dentin of the post channel and the remaining tooth structure. Fig. 12. The use of a small micro-brush greatly facilitates the application of dentin primer and adhesive into the post channel.
Endo-Restorative Interface: Current Concepts 363Fig. 13. A shiny surface confirms an even distribution of the dentin adhesive. 9. Acid-etch the enamel (if present) with 37% phosphoric acid for 30 seconds, and dentin for 15 seconds (Fig. 11).10. Rinse and air-dry.11. Use a small micro-brush to apply a primer that can be used with a self-cure or dual-cure core material to the dentin according to the manufacturer’s instructions (Fig. 12). Gently air-dry.Fig. 14. The use of a needle tube for delivering composite into the post space minimizesvoid formation. The tip of the needle tube is inserted until it reaches the root canal filling.Then the composite is applied from the base of the post channel coronally, and the postspace is filled to the brim. The post is immediately inserted into the composite.
364 Ree Schwartz Fig. 15. A composite core is added to the newly placed post. To prevent bonding core mate- rial to adjacent teeth, as well as to enhance the adaptation of the composite to the remain- ing tooth structure, a core form is used as a matrix. 12. Apply a self-cure or dual-cure dental adhesive that can be used with a self-cure or dual-cure core material to the dentin according to the manufacturer’s instructions (Fig. 13). 13. Inject a self-cure or dual-cure composite in the post space by using a needle tube (Fig. 14). 14. Insert the post into the post channel filled with composite. 15. Use a matrix to prevent bonding core material to adjacent teeth, as well as to enhance the adaptation of the composite to the remaining tooth structure (Fig. 15). Fig. 16. The composite core is added to the newly placed post in a bulk fill, using the same self-curing composite placed in the post channel.
Endo-Restorative Interface: Current Concepts 365Fig. 17. The composite core is contoured and finished.16. Add the remaining composite to the newly placed post or use a light-cure composite for that purpose in increments (Fig. 16).17. Light-cure if necessary, or wait for at least 5 minutes until the self-cure composite has completely set.18. Contour and adjust the occlusion (Fig. 17).19. Finish and polish the restoration.20. Take a final radiograph (Fig. 18).Fig. 18. The radiograph shows a well-adapted fiber post and composite buildup withoutvoids, which is ready to be prepared for a crown.
366 Ree Schwartz SUMMARY AND RECOMMENDATIONS Evaluate restorability carefully before considering endodontic and restorative treatment. Preserve radicular and coronal dentin, especially in the cervical area, to maximize the long-term restorative result. Use adhesive procedures at both radicular and coronal levels to strengthen remaining tooth structure and optimize restoration stability and retention. Use post and core materials with physical properties similar to those of natural dentin. Use a rubber dam when performing clinical procedures involving adhesive bonding. Choose a post that fits passively into the canal preparation. Preserve an apical root canal filling of at least 4 to 5 mm. Use a post length that equals at least the crown height, and that extends apically beyond the crest of bone. Consider placing 2 posts in oval-shaped canals. Consider placing a fiber post through the existing crown in bridge abutments or small teeth. The post will increase crown retention and may improve resis- tance to fracture as long as no additional radicular dentin is removed in the process. With an adequate ferrule and canal thickness, use a fiber post to distribute forces more evenly in the root and reduce the chances of root fracture. If there is an inadequate ferrule, longevity may be compromised, no matter which post is used. Metal posts are stronger and more resistant to flexure, but the stress distribu- tion is unfavorable, with higher risk of root fracture. The stress distribution in fiber posts is more favorable, but these posts are more susceptible to fracture and more likely to flex under load, which may result in micro-movements of the core, and subsequent leakage, caries, and retention loss. REFERENCES 1. Kakehashi S, Stanley HR, Fitzgerald RJ. The effects of surgical exposures of dental pulps in germ-free and conventional laboratory rats. Oral Surg Oral Med Oral Pathol 1965;20:340–9. 2. Siqueira JF Jr. Aetiology of root canal treatment failure and why well-treated teeth can fail. Int Endod J 2001;34:1–10. 3. Saunders WP, Saunders EM. Coronal leakage as a cause of failure in root canal therapy: a review. Endod Dent Traumatol 1994;10:105–8. 4. Alves J, Walton R, Drake D. Coronal leakage: endotoxin penetration from mixed bacterial communities through obturated, post-prepared root canals. J Endod 1998;24:587–91. 5. Ray HA, Trope M. Periapical status of endodontically treated teeth in relation to the technical quality of the root filling and the coronal restoration. Int Endod J 1995;28:12–8. 6. Iqbal MK, Johansson AA, Akeel RF, et al. A retrospective analysis of factors associated with the periapical status of restored, endodontically treated teeth. Int J Prosthodont 2003;16(1):31–8.
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